This invention relates generally to a method and also an apparatus for continuously measuring a specified ingredient contained in a liquid to be tested, and, more particularly, to a method and apparatus wherein accurate measurement of concentration may be performed by compensating the dilution ratio as necessary during continuous measurement of the liquid under test as diluted by a diluent while using a tube pump for feeding the liquid.
It is common, when measuring the concentration of a specified ingredient (the substance to be measured) contained in a liquid to be tested, to supply to the measuring device a mixture of the liquid under test and some other liquid (in other words, the liquid to be tested is diluted th other liquid) and not necessarily a single liquid. There are many purposes for such dilution for example, for pH adjustment, the removal of interfering matter, the prevention of coagulation in a blood sample, and others. All these treatments, however, are liable to cause errors in the measured values of concentration due to errors in or scattering of the mixing rate (dilution ratio). In particular, when continuously selecting the liquid under test which is flowing through a particular channel and whose concentration is continuously monitored, it is often difficult to keep the dilution ratio constant, and the fluctuation is directly expressed in errors of the measured value.
For example, in an apparatus wherein blood is drawn from a vein of a patient by the use of a double current catheter while infusing an anticoagulant in order to continuously and automatically monitor the concentration of a specified ingredient (for example, glucose) contained in the blood, a device for feeding liquid to the blood drawing part of the catheter is generally employed. Such device may be a tube pump having few pulsations and little dead volume that is able to continuously feed a very small quantity of liquid. The discharge quantity of such pumps, however, changes over time and causes the dilution ratio to fluctuate when used for continuous measurement for many hours. Therefore, it is difficult to accurately measure the correct value. This is attributable to deterioration or deformation over time of the pump's tubes because they are continuously squeezed. Also, the inner diameter of the tubes changes due to the adhesion of the constituents of the liquids to the inner tube walls.
In order to avoid these problems and compensate for the change in dilution ratio, heretofore a method has been adopted in which a double current catheter is detached from the patient in the middle of a measurement, and made to draw in a reference liquid. After the dilution ratio is compensated, the catheter is reattached to the patient. When using this method, however, the measurement is not only interrupted but also becomes inconvenient. Further, the patient is susceptible to infection since the catheter must be withdrawn and reinserted.
In order to eliminate these problems, there is disclosed in the Japanese patent application disclosure No. 135795 of 1977, a technique in which the dilution ratio is compensated by comparing the blood separately drawn from a vein of the patient by the use of a syringe with the blood sample drawn from the catheter. However, drawing blood separately by syringe not only causes pain to the patient but also increases the quantity of blood loss. Also, it is required for the blood sample drawn by syringe to be treated with an anticoagulant within a prescribed short time, and to be diluted accurately to a prescribed value. This is a very complex procedure.
As mentioned above, the conventional method of compensating the dilution ratio while continuously drawing blood causes pain and danger of infection to the patient, requires many people and is very complicated, involving the possibility of errors attributable to mistakes in the procedure.